Refrigeration system with fluid defrost

ABSTRACT

A refrigeration system having a refrigerant circuit including a condenser, a flow control device, an evaporator, and a compressor connected in series. The compressor is configured to circulate a cooling fluid through the refrigerant circuit. The refrigerant circuit has an inlet line fluidly connecting the condenser to the evaporator and a suction line fluidly connecting the evaporator to the compressor. A heater is positioned to heat the cooling fluid during a defrost mode, and a pressure control is coupled to the refrigerant circuit downstream of the evaporator. In the defrost mode, the pressure control apparatus is configured to increase system pressure during the defrost mode to maintain flow of refrigerant into the evaporator and to control flow of cooling fluid to the compressor.

BACKGROUND

The present invention relates to refrigeration systems and, more particularly, to fluid defrost of heat exchangers in refrigeration systems.

Refrigeration systems are well known and widely used in supermarkets, warehouses, and elsewhere to refrigerate product that is supported in a refrigerated space. Conventional refrigeration systems include a heat exchanger or evaporator, a compressor, and a condenser. The evaporator provides heat transfer between a refrigerant flowing within the evaporator and a fluid (e.g., water, air, etc.) passing over or through the evaporator. The evaporator transfers heat from the fluid to the refrigerant to cool the fluid. The refrigerant absorbs the heat from the fluid and evaporates in a refrigeration mode, during which the compressor mechanically compresses the evaporated refrigerant from the evaporator and feeds the superheated refrigerant to the condenser, which cools the refrigerant. From the condenser, the cooled refrigerant is typically fed through an expansion valve to reduce the temperature and pressure of the refrigerant, and then the refrigerant is directed through the evaporator.

Some evaporators operate at evaporating refrigerant temperatures that are near or lower than the freezing point of water (i.e., 32 degrees Fahrenheit). Over time, water vapor from the fluid freezes on the evaporator (e.g., on the coils) and generates frost. Accumulation of frost decreases the efficiency of heat transfer between the evaporator and the fluid passing over the evaporator, which causes the temperature of the refrigerated space to increase above a desired level. Maintaining the correct temperature of the refrigerated space is important to maintain the quality of the stored product. To do this, evaporators must be regularly defrosted to reestablish efficiency and proper operation. Many existing refrigeration systems use electric heaters that are placed underneath the evaporator to defrost the evaporator using convection heat. Other existing systems re-route hot gaseous refrigerant from the compressor directly to the evaporator so that heat from the hot refrigerant melts the frost on the evaporator (i.e. reverse hot gas defrost).

SUMMARY

In one aspect, the present invention provides a refrigeration system having a refrigerant circuit including a condenser, a flow control device, an evaporator, and a compressor connected in series. The compressor is configured to circulate a cooling fluid through the refrigerant circuit. The refrigerant circuit has an inlet line fluidly connecting the condenser to the evaporator and a suction line fluidly connecting the evaporator to the compressor. A heater is positioned to heat the cooling fluid during a defrost mode, and a pressure control is coupled to the refrigerant circuit downstream of the evaporator. In the defrost mode, the pressure control apparatus is configured to increase system pressure during the defrost mode to maintain flow of refrigerant into the evaporator and to control flow of cooling fluid to the compressor.

In another aspect, the present invention provides a refrigeration system having a refrigerant circuit including a condenser, a flow control device, an evaporator, and a compressor connected in series. The compressor is configured to circulate a refrigerant through the refrigerant circuit. The refrigerant circuit has an inlet line fluidly connecting the condenser to the evaporator and a suction line fluidly connecting the evaporator to the compressor. A refrigeration mode directs the refrigerant through the evaporator in a first direction via the inlet line. A defrost mode directs refrigerant though the evaporator in the first direction via the inlet line. A first heater is coupled to the inlet line and configured to heat the refrigerant during the defrost mode. A pressure control apparatus is coupled to the refrigerant circuit downstream of the evaporator and configured to increase system pressure to maintain flow of refrigerant into the evaporator during the defrost mode.

In another aspect, the present invention provides a method of defrosting a refrigeration system having a refrigeration mode and a defrost mode. The method includes circulating refrigerant in the refrigeration mode through a condenser, a flow control device, an evaporator, and a compressor of the refrigeration system. The method also includes circulating refrigerant in the defrost mode through the evaporator in the same direction as the refrigeration mode. The method also includes heating the refrigerant in the defrost mode upstream of the evaporator and increasing the system pressure in the defrost mode to maintain flow of refrigerant into the evaporator during the defrost mode.

Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of a refrigerated merchandiser including a product display area and an evaporator that is disposed in a refrigerant circuit of a refrigeration system embodying the present invention.

FIG. 2a is a schematic view of an exemplary refrigeration system that is in a refrigeration mode and that includes a refrigerant circuit with the evaporator, pressure control apparatus, a compressor, a condenser, and a flow control device fluidly arranged in series with each other, the refrigeration system also including a heater that is disposed upstream of the evaporator.

FIG. 2b is a schematic view of the refrigeration system of FIG. 2a in a defrost mode including a heater that is disposed upstream of the evaporator and a pressure control apparatus.

FIG. 3 is a schematic view of another exemplary refrigeration system that is in a defrost mode and that includes the first heater disposed upstream of the evaporator, a second heater disposed downstream of the evaporator, and a first and second pressure regulator.

FIG. 4 is a schematic view of another exemplary refrigeration system that is in a defrost mode and that includes a first heater disposed upstream of the evaporator, a second heater disposed downstream of the evaporator, and a solenoid valve.

FIG. 5 is a schematic view of another exemplary refrigeration system that is in a defrost mode and that includes a first heater disposed upstream of the evaporator, a second heater disposed downstream of the evaporator, and an electronic pressure regulator.

FIG. 6 is a schematic view of another exemplary refrigeration system that is in a defrost mode and that includes a heater that is disposed upstream of the evaporator, the flow control device including an electronic expansion valve, and first and second pressure regulators.

FIG. 7 is a schematic view of another exemplary refrigeration system that is in a defrost mode and that includes a heater that is disposed upstream of the evaporator, the flow control device including the electronic expansion valve, and the pressure control apparatus including a solenoid.

FIG. 8 is a schematic view of another exemplary refrigeration system that is in a defrost mode and that includes the heater that is disposed upstream of the evaporator, the flow control device including the electronic expansion valve, and the pressure control apparatus including an electronic pressure regulator.

FIG. 9a is a schematic view of another exemplary refrigeration system that is in a refrigeration mode and that includes a refrigerant circuit similar to the circuit illustrated in FIG. 2a and including a recirculation line.

FIG. 9b is a schematic view of the refrigeration system of FIG. 9a in a defrost mode that is in a first state.

FIG. 9c is a schematic view of the refrigeration system of FIG. 9a in the defrost mode that is in a second state.

Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways.

DETAILED DESCRIPTION

FIG. 1 illustrates a refrigerated merchandiser 10 that may be located in a supermarket or a convenience store (not shown) for presenting fresh food, beverages, and other food product to consumers. The merchandiser 10 includes a case 15 that has a base 20, a rear wall 25, a canopy 30, and an opening 35 allowing access to the food product. The area partially enclosed by the base 20, the rear wall 25, and the canopy 30 defines a product display area 40 for supporting the food product in the case 15. For example, food product can be displayed on racks or shelves 43 extending forward from the rear wall 25, and is accessible by consumers through the opening 35 adjacent the front of the case 15. Although the merchandiser 10 is illustrated as an open-front merchandiser, the merchandiser can include doors.

With reference to FIGS. 1 and 2 a, the exemplary refrigerated merchandiser 10 has a portion of a refrigeration system 45 that is in communication with the case 15 to provide a refrigerated airflow to the product display area 40. The refrigeration system 45 illustrated in FIG. 2a is a generic, exemplary system and includes a refrigerant circuit 47 that has a condenser 50, a flow control device 55, an evaporator 60, and a compressor 65 connected in series. The refrigerant circuit 47 has an inlet line 85 that fluidly connects the condenser 50 to the evaporator 60, and a suction line 90 that fluidly connects the evaporator 60 to the compressor 65. The flow control device 55 is disposed in the inlet line 85 and controls refrigerant flow to the evaporator 60 (and thus, the superheat at the evaporator outlet). The refrigerant circuit 47 also has a heater 75 (e.g., a ceramic heater, an induction heater, etc.) that is coupled to the inlet line 85 (illustrated downstream of the flow control device 55) upstream of the evaporator 60, and pressure control apparatus 80 that is disposed in the suction line 90. Referring back to FIG. 1, the evaporator 60 is disposed in an air passageway 70 of the merchandiser 10 to condition air that is directed through the air passageway 70.

The refrigeration system 45 has a refrigeration mode during which the evaporator 60 conditions an airflow (e.g., the air flowing through passageway 70 in the merchandiser 10) based on heat transfer between the refrigerant in the evaporator 60 and the air passing over the evaporator 60. The refrigeration system also has a defrost mode during which frost buildup on the evaporator 60 is reduced or removed.

Although the invention is described with reference to its application in the refrigerated merchandiser 10, it will be appreciated that the refrigeration system 45 and method for defrosting the refrigeration system 45 described in detail below will have other applications.

FIG. 2a illustrates a schematic view of the refrigeration system 45 in the refrigeration mode. In the refrigeration mode, the compressor 65 is configured to circulate high pressure gaseous cooling fluid or refrigerant (described as “refrigerant” for purposes of description) to the condenser 50. The condenser 50 rejects heat from the compressed hot gas, causing the refrigerant to condense into high pressure liquid. The condensed refrigerant is directed through the inlet line 85 as a liquid to the flow control device 55, which expands the refrigerant into a low pressure (e.g., saturated) vapor refrigerant. The saturated refrigerant is evaporated as it passes through the evaporator 60 as a result of absorbing heat from air passing over the evaporator 60. The absorption of heat by the refrigerant allows the temperature of the airflow to decrease as it passes over the evaporator 60. The heated or gaseous refrigerant then exits the evaporator 60 and is directed back to the compressor 65 through the suction line 90 for re-processing through the refrigeration system 45. In the exemplary merchandiser 10, the cooled or refrigerated airflow exiting the evaporator 60 via heat exchange with the liquid refrigerant is directed through the remainder of the air passageway 70 and is introduced into the product display area 40 where the airflow will remove heat from and maintain the food product at desired conditions.

As shown in FIGS. 2a and 2b , the refrigeration system 45 also includes a sensor 95 (e.g., a defrost termination sensor), that is coupled to the evaporator 60, and a controller 100 that is communication (wired or wirelessly) with the sensor 95 and other electric or electronically-controlled devices (e.g., the flow control device 55, the compressor 65, the heater 75, the pressure control apparatus 80, etc.). The sensor 95 is coupled to the evaporator 60 to detect the temperature of refrigerant exiting the evaporator 60. It will be appreciated that the sensor 95 can be coupled to the suction line 90 just downstream of the evaporator 60 and accomplish the same task. The controller 100 is programmed to receive a signal from the sensor 95 that is indicative of the evaporator-refrigerant temperature, and to control various devices based on the signal. The controller 100 also can control other components of the system 45 based on or independent of the signal.

FIG. 2b illustrates a schematic view of the refrigeration system 45 in the defrost mode. The sensor 95 can be used to detect buildup of frost on the evaporator 60 based on the temperature of the refrigerant exiting the evaporator 60, although frost buildup can be detected in other ways. In some constructions, the controller 100 automatically initiates the defrost mode at preset time periods (e.g., every two hours) or preset times (e.g., 2 a.M.).

In the defrost mode, the controller 100 activates the heater 75, which begins heating the refrigerant flowing to the evaporator 60. The flow control device 55 regulates (e.g., maintains, increases, or decreases) the flow of refrigerant to the evaporator 60 during the defrost mode, and ensures that refrigerant continues to flow to the evaporator 60 in the defrost mode. The pressure control apparatus 80 is configured to increase system pressure during the defrost mode to maintain flow of refrigerant into the evaporator 60 and to control flow of refrigerant to the compressor 65. As illustrated in FIG. 2b , refrigerant continues to flow to the compressor 65 during the defrost mode. In general, the pressure control apparatus 80 increases the amount of refrigerant mass in the evaporator 60 while controlling backfeeding of liquid refrigerant to the compressor 65. The constant flow of the heated refrigerant during the defrost mode increases the temperature of the evaporator 60 and melts frost on the exterior of the evaporator 60.

The defrost mode is deactivated by the controller 100 in response to the sensor 95 detecting a refrigerant temperature at or above a predetermined temperature threshold, and the refrigeration system 45 returns to the refrigeration mode. The heater 75 is turned off at this transition between modes, and the controller 100 can control the pressure control apparatus 80 to regulate how much refrigerant flows to the compressor 65.

FIG. 3 illustrates a schematic view of an exemplary refrigeration system 145 in the defrost mode. The refrigeration system 145 incorporates the components of the refrigeration system 45, and these components are labeled with the same reference numerals. The flow control device 55 includes a mechanical thermal expansion valve 130 and a second heater 125 that is in communication with the suction line 90 to control the opening amount of the thermal expansion valve 130 based on the temperature of refrigerant at or adjacent the evaporator outlet. More specifically, the second heater 125 controls the dry bulb temperature and can be controlled to heat the thermal expansion valve 130 to feed refrigerant (or maintain refrigerant flow) to the evaporator during the defrost mode.

The pressure control apparatus 80 for the refrigeration system 145 includes a first pressure regulator 105 that is disposed in the suction line 90, and a solenoid valve 120 and a second pressure regulator 115 that are disposed in a bypass line 110 that bypasses the first pressure regulator 105. The first pressure regulator 105 (e.g., an evaporator pressure regulator valve) has a first pressure setpoint (e.g., 30 psi) and the second pressure regulator 115 (e.g., an evaporator pressure regulator valve) has a second pressure setpoint (e.g., 130 psi) that is higher than the first pressure setpoint.

The solenoid valve 120 controls the flow of refrigerant in the bypass line 110 and to the second pressure regulator 115. During the refrigeration mode (not shown) for the refrigeration system 145, the solenoid valve 120 is closed. During the defrost mode, the solenoid valve 120 is open and the first and second pressure regulators 105, 115 build pressure in the system 145 to maximize the amount of refrigerant in the evaporator 60. When the system pressure exceeds the first pressure setpoint, refrigerant begins to flow to the compressor 65 through the first pressure regulator 105 and the suction line 90. Between the first pressure setpoint and the second pressure setpoint, refrigerant does not flow through the bypass line 110 to the compressor 65. Refrigerant only flows through the bypass line 110 when the system pressure exceeds the second pressure setpoint. The defrost mode is terminated when the sensor 95 detects an evaporator temperature that exceeds the predetermined threshold.

FIG. 4 illustrates a schematic view of an exemplary refrigeration system 245 in the defrost mode. The refrigeration system 245 incorporates the components of the refrigeration system 45. The flow control device 55 includes a mechanical thermal expansion valve 230 and a second heater 225 that is in communication with the suction line 90 a second heater 225 that is in communication with the suction line 90 to control the opening amount of the thermal expansion valve 130 based on the temperature of refrigerant at or adjacent the evaporator outlet. More specifically, the second heater 225 controls the dry bulb temperature and can be controlled to heat the thermal expansion valve 230 to feed refrigerant (or maintain refrigerant flow) to the evaporator during the defrost mode. The pressure control apparatus 80 for the refrigeration system 245 includes a solenoid valve 235 that is disposed in the suction line 90. The solenoid valve 235 is in communication with the controller 100 so that the controller 100 can open and close the solenoid valve 235.

The solenoid valve 235 controls the flow of refrigerant in the suction line 90 by permitting or inhibiting refrigerant flow. During the refrigeration mode (not shown) for refrigeration system 245, the solenoid valve 235 is open. During the defrost mode, the solenoid valve 235 is closed and builds pressure in the system 245 to maximize the amount of refrigerant in the evaporator 60. If the pressure of refrigerant in the system 245 exceeds a safety pressure setpoint (e.g., 130 psi), the controller 100 opens the solenoid valve 235 and refrigerant begins to flow to the compressor 65 through the suction line 90. The defrost mode is terminated when the sensor 95 detects an evaporator temperature that exceeds the predetermined threshold.

FIG. 5 illustrates a schematic view of an exemplary refrigeration system 345 in the defrost mode. The refrigeration system 345 incorporates the components of the refrigeration system 45. The flow control device 55 includes a mechanical thermal expansion valve 330 and a second heater 325 that is in communication with the suction line 90 to control the opening amount of the thermal expansion valve 330 based on the temperature of refrigerant at or adjacent the evaporator outlet. More specifically, the second heater 325 controls the dry bulb temperature and can be controlled to heat the thermal expansion valve 130 to feed refrigerant (or maintain refrigerant flow) to the evaporator during the defrost mode. The pressure control apparatus 80 for the refrigeration system 345 includes an electronic pressure regulator 340 (e.g., an electronic evaporator pressure regulator valve) that is disposed in the suction line 90. The electronic pressure regulator 340 is in communication with the controller 100.

The electronic pressure regulator 340 controls the flow of refrigerant in the suction line 90. During the refrigeration mode (not shown) for refrigeration system 345, the electronic pressure regulator 340 is open. During the defrost mode, the electronic pressure regulator 340 is controlled to build pressure in the system 345 to maximize the amount of refrigerant in the evaporator 60. When a predetermined pressure setpoint or threshold (e.g., 30 psi, 130 psi, etc.) is exceeded, the electronic pressure regulator 340 at least partially opens and releases refrigerant to the compressor 65 through the suction line 90 to relieve pressure in the system 345. The defrost mode is terminated when the sensor 95 detects an evaporator temperature that exceeds the predetermined threshold.

FIG. 6 illustrates a schematic view of an exemplary refrigeration system 445 in the defrost mode. The refrigeration system 445 incorporates the components of the refrigeration system 45. The flow control device 55 includes an electronic expansion valve 460 that is in communication with the controller 100 that determines the opening amount of the electronic expansion valve 460 based on the temperature of the refrigerant at or adjacent the evaporator outlet. The pressure control apparatus 80 for the refrigeration system 445 includes a first pressure regulator 405 that is disposed in the suction line 90, and a solenoid valve 420 and a second pressure regulator 415 that are disposed in a bypass line 410 that bypasses the first pressure regulator 105. The first pressure regulator 405 (e.g., an evaporator pressure regulator valve) has a first pressure setpoint (e.g., 30 psi) and the second pressure regulator 415 (e.g., an evaporator pressure regulator valve) has a second pressure setpoint (e.g., 130 psi) that is higher than the first pressure setpoint.

The solenoid valve 420 controls the flow of refrigerant in the bypass line 410 and to the second pressure regulator 415. During the refrigeration mode (not shown) for the refrigeration system 445, the solenoid valve 420 is closed. During the defrost mode, the solenoid valve 420 is open and the first and second pressure regulators 405, 415 build pressure in the system 445 to maximize the amount of refrigerant in the evaporator 60. When the system pressure exceeds the first pressure setpoint, refrigerant begins to flow to the compressor 65 through the first pressure regulator 405 and the suction line 90. Between the first pressure setpoint and the second pressure setpoint, refrigerant does not flow through the bypass line 410 to the compressor 65 due to the higher pressure setpoint of the second pressure regulator 415. Refrigerant only flows through the bypass line 410 when the system pressure exceeds the second pressure setpoint. The defrost mode is terminated when the sensor 95 detects an evaporator temperature that exceeds the predetermined threshold.

FIG. 7 illustrates a schematic view of an exemplary refrigeration system 545 in the defrost mode. The refrigeration system 545 incorporates the components of the refrigeration system 45. The flow control device 55 includes an electronic expansion valve 560 in communication with the controller 100 that determines the opening amount of the electronic expansion valve 560 based on the temperature of the refrigerant at or adjacent the evaporator outlet. The pressure control apparatus 80 for the refrigeration system 545 includes a solenoid valve 535 that is disposed in the suction line 90. The solenoid valve 535 is in communication with controller 100.

The solenoid valve 535 controls the flow of refrigerant in the suction line 90. During the refrigeration mode (not shown) for the refrigeration system 545, the solenoid valve 535 is open. During the defrost mode, the solenoid valve 535 is closed and builds pressure in the system 545 to maximize the amount of refrigerant in the evaporator 60. If the pressure of refrigerant in the system 545 exceeds a safety pressure setpoint (e.g., 130 psi), the controller 100 opens the solenoid valve 535 and refrigerant begins to flow to the compressor 65 through the suction line 90. The defrost mode is terminated when the sensor 95 detects an evaporator temperature that exceeds the predetermined threshold.

FIG. 8 illustrates a schematic view of an exemplary refrigeration system 645 in the defrost mode. The refrigeration system 645 incorporates the components of the refrigeration system 45. The flow control device 55 includes an electronic expansion valve 660 in communication with the controller 100 that determines the opening amount of the electronic expansion valve 660 based on the temperature of the refrigerant at or adjacent the evaporator outlet. The pressure control apparatus 80 for the refrigeration system 645 includes an electronic pressure regulator 640 (e.g., an electronic evaporator pressure regulator valve) that is disposed in the suction line 90. The electronic pressure regulator 640 is in communication with the controller 100.

The electronic pressure regulator 640 controls the flow of refrigerant in the suction line 90. During the refrigeration mode (not shown) for refrigeration system 645, the electronic pressure regulator 640 is open. During the defrost mode, the electronic pressure regulator 640 is controlled by the controller 100 to build pressure in the system 645 to maximize the amount of refrigerant in the evaporator 60. The electronic pressure regulator 640 releases refrigerant through the suction line 90 to the compressor 65 to relieve pressure in the system 645 based on the pressure setpoint of the pressure regulator 640. The defrost mode is terminated when the sensor 95 detects an evaporator temperature that exceeds the predetermined threshold.

FIG. 9a illustrates a schematic view of an exemplary refrigeration system 745 in the refrigeration mode. The refrigeration system 745 incorporates the components of the refrigeration system 45. The refrigerant circuit further includes a recirculation line 750 that is fluidly connected between the suction line 90 and the inlet line 85, and a solenoid valve 755 that is disposed in the recirculation line 750. During the refrigeration mode for the refrigeration system 745, the solenoid valve 755 is closed, and refrigerant circulates through the circuit 747 consistent with what has been described with regard to FIG. 2 a.

FIGS. 9b and 9c illustrate a schematic view of the refrigeration system 745 of FIG. 9a in the defrost mode. During the defrost mode, the solenoid valve 755 is opened (via the controller 100) and refrigerant recirculates through the recirculation line 750 from the suction line 90 (upstream of the pressure control apparatus 80) to the inlet line 85 upstream of the heater 75. As illustrated in FIG. 9b , the pressure of the refrigerant exiting the evaporator 60 is below the pressure setpoint of the pressure control apparatus 80. As a result, system refrigerant pressure increases (i.e. no refrigerant flows to the compressor in this initial cycle of the defrost mode) and the heated refrigerant directed into the evaporator 60 is only recirculated to the inlet line 85 upstream of the heater 75, where the refrigerant is again heated and directed through the evaporator 60 for further defrost operations.

At some point, as illustrated in FIG. 9c , the system pressure will likely exceed the pressure setpoint of the pressure control apparatus 80. When this happens, refrigerant in the circuit during the defrost mode 1) recirculates from the evaporator outlet to the inlet line 85 upstream of the heater 75, and 2) flows to the compressor 65 through the suction line 90 to relieve system pressure.

As illustrated by the generic diagrams of FIGS. 2a and 2b , and by the several exemplary refrigeration systems illustrated in FIGS. 3-9 c, the evaporator 60 can be defrosted by placing a heater upstream of the evaporator inlet without having to reverse the flow of refrigerant within the system. That is, the systems illustrated and described relative to the Figures do not require the complex refrigerant piping of a reverse hot-gas defrost system that pipes heated refrigerant from the compressor 65 through a separate line to the evaporator 60. In addition, the heater 75 directly heats the refrigerant, which reduces the amount of time needed to start and end defrost of the evaporator 60 compared to existing systems. The flow control device and the pressure control apparatus keep the evaporator sufficiently hot to facilitate defrost by controlling the pressure of the system while also keeping active refrigerant circulation within the circuit.

Various features and advantages of the invention are set forth in the following claims. 

1. A refrigeration system comprising: a refrigerant circuit including a condenser, a flow control device, an evaporator, and a compressor connected in series, the compressor configured to circulate a cooling fluid through the refrigerant circuit, and the refrigerant circuit having an inlet line fluidly connecting the condenser to the evaporator and a suction line fluidly connecting the evaporator to the compressor; a heater positioned to heat the cooling fluid during a defrost mode; and pressure control apparatus coupled to the refrigerant circuit downstream of the evaporator, wherein, in the defrost mode, the pressure control apparatus is configured to increase system pressure during the defrost mode to maintain flow of refrigerant into the evaporator and to control flow of cooling fluid to the compressor.
 2. The refrigeration system of claim 1, wherein the heater is positioned downstream of the flow control device.
 3. The refrigeration system of claim 1, wherein the pressure control apparatus includes a solenoid valve or a pressure regulator.
 4. The refrigeration system of claim 1, wherein the pressure control apparatus includes a first pressure regulator coupled to the suction line, the refrigerant circuit further including a bypass line that bypasses the first pressure regulator and that has a second pressure regulator, and wherein the first pressure regulator has a first pressure setpoint and the second pressure regulator has a second pressure setpoint that is higher than the first pressure setpoint.
 5. The refrigeration system of claim 4, wherein the first pressure regulator includes an evaporator pressure regulator valve and the second pressure regulator includes a solenoid valve.
 6. The refrigeration system of claim 1, wherein the heater includes a first heater, the refrigeration system further comprising a second heater in communication with the flow control device, wherein the second heater is configured to control the flow control apparatus to selectively permit or restrict flow of cooling fluid to the evaporator during the defrost mode.
 7. The refrigeration system of claim 1, further comprising a sensor coupled to the evaporator and configured to detect a temperature of the evaporator, and a controller in communication with the heater to activate the heater in response to buildup of frost on the evaporator, wherein the controller is configured to terminate the defrost mode by deactivating the heater in response to the sensor detecting a cooling fluid temperature at or above a predetermined temperature threshold.
 8. The refrigeration system of claim 7, wherein the temperature of the evaporator detected by the sensor includes a temperature of the cooling fluid in or exiting the evaporator.
 9. The refrigeration system of claim 7, wherein the controller is further in communication with the pressure control apparatus to regulate a position of the pressure control apparatus between an open position and a closed position.
 10. The refrigeration system of claim 1, wherein the refrigeration circuit further includes a recirculation line fluidly connected between the suction line and the inlet line, wherein the recirculation line is configured to recirculate cooling fluid exiting the evaporator to the inlet line upstream of the heater.
 11. A refrigeration system comprising: a refrigerant circuit including a condenser, a flow control device, an evaporator, and a compressor connected in series, the compressor configured to circulate a refrigerant through the refrigerant circuit, and the refrigerant circuit having an inlet line fluidly connecting the condenser to the evaporator and a suction line fluidly connecting the evaporator to the compressor; a refrigeration mode in which refrigerant is directed through the evaporator in a first direction via the inlet line; a defrost mode in which refrigerant is directed through the evaporator in the first direction via the inlet line; a first heater coupled to the inlet line and configured to heat the refrigerant during the defrost mode; and a pressure control apparatus coupled to the refrigerant circuit downstream of the evaporator and configured to increase system pressure to maintain flow of refrigerant into the evaporator during the defrost mode.
 12. The refrigeration system of claim 11, wherein the heater is positioned downstream of the flow control device.
 13. The refrigeration system of claim 11, wherein the pressure control apparatus includes a solenoid valve or a pressure regulator.
 14. The refrigeration system of claim 11, wherein the pressure control apparatus includes a first pressure regulator coupled to the suction line, the refrigerant circuit further including a bypass line that bypasses the first pressure regulator and that has a second pressure regulator, and wherein the first pressure regulator has a first pressure setpoint and the second pressure regulator has a second pressure setpoint that is higher than the first pressure setpoint.
 15. The refrigeration system of claim 14, wherein the first pressure regulator includes an evaporator pressure regulator valve and the second pressure regulator includes a solenoid valve.
 16. The refrigeration system of claim 11, wherein the heater includes a first heater, the refrigeration system further comprising a second heater in communication with the flow control device, wherein the second heater is configured to control the flow control apparatus to permit flow of cooling fluid to the evaporator during the defrost mode.
 17. The refrigeration system of claim 11, further comprising a sensor coupled to the evaporator and configured to detect a temperature of the evaporator, and a controller in communication with the heater to activate the heater in response to buildup of frost on the evaporator, wherein the controller is configured to terminate the defrost mode by deactivating the heater in response to the sensor detecting a cooling fluid temperature at or above a predetermined temperature threshold.
 18. The refrigeration system of claim 17, wherein the controller is further in communication with the pressure control apparatus to regulate a position of the pressure control apparatus between an open position and a closed position.
 19. The refrigeration system of claim 11, wherein the refrigeration circuit further includes a recirculation line fluidly connected between the suction line and the inlet line, wherein the recirculation line is configured to recirculate cooling fluid exiting the evaporator to the inlet line upstream of the heater.
 20. A method of defrosting a refrigeration system having a refrigeration mode and a defrost mode, the method comprising: circulating refrigerant in the refrigeration mode through a condenser, a flow control device, an evaporator, and a compressor of the refrigeration system; circulating refrigerant in the defrost mode through the evaporator in the same direction as the refrigeration mode; heating the refrigerant in the defrost mode upstream of the evaporator; and increasing system pressure in the defrost mode to maintain flow of refrigerant into the evaporator during the defrost mode.
 21. The method of claim 20, further comprising deactivating the defrost mode based on a temperature of refrigerant exiting the evaporator meeting or exceeding a temperature threshold. 